| dbo:abstract
|
- التناظر المركزي (Centrosymmetry) في علم البلورات هي خاصية تمتلكها أي زمرة نقطية عندما يكون لها مركز انقلاب كأحد عناصر التناظر. يمكن أن يكون للبلورات التي لها مركز انقلاب خصائص مميزة مثل الكهرباء الانضغاطية. (ar)
- In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements. In such a point group, for every point (x, y, z) in the unit cell there is an indistinguishable point (-x, -y, -z). Such point groups are also said to have inversion symmetry. Point reflection is a similar term used in geometry.Crystals with an inversion center cannot display certain properties, such as the piezoelectric effect. The following space groups have inversion symmetry: the triclinic space group 2, the monoclinic 10-15, the orthorhombic 47-74, the tetragonal 83-88 and 123-142, the trigonal 147, 148 and 162-167, the hexagonal 175, 176 and 191-194, the cubic 200-206 and 221-230. Point groups lacking an inversion center (non-centrosymmetric) can be polar, chiral, both, or neither. A polar point group is one whose symmetry operations leave more than one common point unmoved. A polar point group has no unique origin because each of those unmoved points can be chosen as one. One or more unique polar axes could be made through two such collinear unmoved points. Polar crystallographic point groups include 1, 2, 3, 4, 6, m, mm2, 3m, 4mm, and 6mm. A chiral (often also called enantiomorphic) point group is one containing only proper (often called "pure") rotation symmetry. No inversion, reflection, roto-inversion or roto-reflection (i.e., improper rotation) symmetry exists in such point group. Chiral crystallographic point groups include 1, 2, 3, 4, 6, 222, 422, 622, 32, 23, and 432. Chiral molecules such as proteins crystallize in chiral point groups. The remaining non-centrosymmetric crystallographic point groups 4, 42m, 6, 6m2, 43m are neither polar nor chiral. (en)
- La centrosymétrie est, en cristallographie, la propriété d'une structure cristalline ainsi que de son groupe de symétrie (groupe ponctuel, groupe d'espace) qui contient une symétrie centrale. Dans une telle structure groupe ponctuel, pour chaque point (x, y, z) il existe un point non discernable (-x, -y, -z). Les cristaux qui possèdent un centre d'inversion ne peuvent présenter certaines propriétés, telles que l'effet piézoélectrique. (fr)
- Un oggetto o una struttura cristallina è detta centrosimmetrica se gode di simmetria per inversione spaziale rispetto a un punto, ovvero l'oggetto rimane inalterato applicando l'operatore di parità spaziale: il punto dell'oggetto nelle coordinate (x,y,z) riferite al centro di simmetria è uguale al punto in (-x,-y,-z). Il termine è usato soprattutto in cristallografia. Se una struttura cristallina è centrosimmetrica, allora è incompatibile con una polarizzazione elettrica permanente (ferroelettricità) in quanto il campo elettrico non gode di simmetria per inversione spaziale (applicando l'operatore di parità spaziale infatti, il vettore campo elettrico E va in -E). (it)
- 在数学中，中心对称是几何图形的一种性质。 (zh)
|
| rdfs:comment
|
- التناظر المركزي (Centrosymmetry) في علم البلورات هي خاصية تمتلكها أي زمرة نقطية عندما يكون لها مركز انقلاب كأحد عناصر التناظر. يمكن أن يكون للبلورات التي لها مركز انقلاب خصائص مميزة مثل الكهرباء الانضغاطية. (ar)
- La centrosymétrie est, en cristallographie, la propriété d'une structure cristalline ainsi que de son groupe de symétrie (groupe ponctuel, groupe d'espace) qui contient une symétrie centrale. Dans une telle structure groupe ponctuel, pour chaque point (x, y, z) il existe un point non discernable (-x, -y, -z). Les cristaux qui possèdent un centre d'inversion ne peuvent présenter certaines propriétés, telles que l'effet piézoélectrique. (fr)
- 在数学中，中心对称是几何图形的一种性质。 (zh)
- In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements. In such a point group, for every point (x, y, z) in the unit cell there is an indistinguishable point (-x, -y, -z). Such point groups are also said to have inversion symmetry. Point reflection is a similar term used in geometry.Crystals with an inversion center cannot display certain properties, such as the piezoelectric effect. Point groups lacking an inversion center (non-centrosymmetric) can be polar, chiral, both, or neither. (en)
- Un oggetto o una struttura cristallina è detta centrosimmetrica se gode di simmetria per inversione spaziale rispetto a un punto, ovvero l'oggetto rimane inalterato applicando l'operatore di parità spaziale: il punto dell'oggetto nelle coordinate (x,y,z) riferite al centro di simmetria è uguale al punto in (-x,-y,-z). Il termine è usato soprattutto in cristallografia. (it)
|
